This invention relates to a permanent magnet DC brushless motor and more particularly one of the 2-phase types.
It has been proposed in many applications where accessories or auxiliaries were driven by an internal combustion engine, such as motor vehicles, to drive some or all of these accessories by DC motors. For example, the use of electrically driven hydraulic power steering apparatus has been proposed.
Conventionally, DC motors have been widely used that employ brushes and commutaters. There are a number of disadvantages with this type of mechanism. For example, the operation of the brush with the commutator causes not only wear of the brushes but also wear of the comutator. This also causes the generation of sparks, which can cause noises both electrical and audible. Obviously, this type of mechanism requires servicing of the brushes and commutators and there is a limited operational speed for this and other reasons.
It has been proposed, therefore, to provide a brushless type of DC motor. Generally, this type of motor includes a rotor and an armature that cooperate with each other. However, in order to detect and control the rotational element of the rotor, semi-conductor devices, often referred to as inadverters, are used to accomplish switching of current through each armature coil. Generally, 3-phase units are employed and there is a number of windings frequently associated with each phase. The invertors or, as hereinafter identified, main switch devices, are pulsed with modulated controlled (PWN) with either sign wave (180xc2x0 electrification) or rectangular wave (120xc2x0 electrification) control. As may be seen in FIG. 1, which is a graphical and schematic representation of a 3-phase machine of this type, an electrical battery or other DC power source indicated at 11 is employed for powering the various windings of the motor 12 through an inverter switching bridge circuit 14. Such a system requires at the minimum six switching devices or inverters or solid-state devices S1, S2, S3, S4, S5, and S6. It should be noted that these motors are designed so as to rotate in opposite directions frequently. These switching devices generally are bipolar transistors or the like. Agate circuit, which is not shown, is employed for turning on and off these main switching devices based upon the rotational angle of the rotor so as to supply winding circuit tone or more of the 3-phase windings. The disadvantage with this type of arrangement is that it requires a minimum of six main switching devices. The current supplied to any one winding is via two main switching devices and a voltage drop occurs across each of them. Therefore, as the main switching devices that conduit the winding current are increased in number, the voltage applied to the corresponding winding decreases. This has a distinct disadvantage, particularly in the case of motor vehicles where relatively low voltage battery power sources are employed. The voltage drop caused by the main switching devices becomes relatively large in relation to the source voltage and motor performance decreases.
As a further disadvantage, if the main switching devices of the inverter are driven by 180xc2x0 electrified sign waves, the maximum value of current through the main switching devices reaches the square root of 2 times the effective value of the current. Therefore, the rating of the main switching devices must be raised and cost increases.
There are inverters that are driven by 120xc2x0 electrified rectangular waves. Inverters of this type, however, the torque fluxuations become large and vibrations and undesirable sounds increase. Furthermore, since the driving efficiency of the motor is low, the amount of heat generated is great so that it is difficult to reduce the size of the inverters.
It is, therefore, a principal object to this invention to provide an improved brushless DC electric motor that reduces the number of solid state switching devices required and also thus permits a greater performance. FIGS. 2 and 3 show schematically a device of the type, which may be utilized to improve the performance and reduce the number of switching devices in that it uses a 2-phase system. However, as will become apparent as this description proceeds, this device still has disadvantages in that it requires multiple switching devices each for the respective winding. The remaining figures show embodiments that further reduce the number of switching devices.
It is, therefore, a still further object to this invention to provide an improved 2-phase multiple winding brushless electric motor that has a reduced number of switching devices and thus, greater performance and lower costs.
A first feature of this invention is adapted to be embodied in a brushless type, 2-phase, multiple winding electric motor. The electric motor includes 2 phases each of which has two xe2x80x9cnxe2x80x9d windings and xe2x80x9cNxe2x80x9d as a whole integer. These windings are associated with permanent magnets that circumferentially spaced and are switched by switching devices so as to alter the current flow through them to drive the rotor. The switching devices are operated from a direct current power source and controlled by a respective main switching device. The current value flowing through the windings of each phase is detected by a CT device.
In accordance with another feature of the invention, there is a plurality of windings for each phase. In other words, xe2x80x9cNxe2x80x9d is greater than 1 and the windings are paired so that a single switch controls the flow through two windings and a single CT detects the flow of current through the paired windings.